MINDS: A transition from H₂O to C₂H₂ dominated disk spectra with decreasing stellar luminosity

Context . The chemical composition of the inner regions of disks around young stars will largely determine the properties of planets that form in these regions. Many physical processes in the disks drive their chemical evolution, and some of them depend on and/or correlate with the stellar properties. Aims . We explore the connection between stellar properties and the chemistry of the inner disk in protoplanetary disks as traced by mid-infrared spectroscopy. Methods . We used JWST-MIRI observations of a large diverse sample of sources to explore trends between the carbon-bearing molecule C 2 H 2 and the oxygen-bearing molecule H 2 O. Additionally, we calculated the average spectrum for the T Tauri (M * >0.2 M ⊙ ) and very low-mass star (VLMS; M * ,≤0.2 M ⊙ ) samples from JWST-MIRI MRS data and used slab models to determine the properties of the average spectra in each subsample. Results . We find a significant anticorrelation between the flux ratio of C 2 H 2 /H 2 O and the stellar luminosity. The F C 2 H 2 / F H 2 O flux ratios of disks around VLMSs are significantly higher than the fluxes in their higher-mass counterparts. This is driven by the generally weak H 2 O and strong C 2 H 2 in disks around low-mass hosts. We also explored trends with the strength of the 10 µm silicate feature, the stellar accretion rate, and the disk dust mass. They are all correlated with F C 2 H 2 / F H 2 O , which may be related to processes that drive the carbon enrichment in disks around VLMSs, but are also degenerate with the system properties (i.e., the M * − Ṁ and M * − M disk relations). Slab model fits to the average spectra show that H 2 O emission in the VLMS sample is quite similar in temperature and column density to a warm (~600 K) H 2 O component in the T Tauri spectrum. This indicates that the high C/O gas-phase ratio in these disks is not due to oxygen depletion alone. Instead, the many hydrocarbons, including some with high column densities, suggest that carbon enhancement occurs in the disks around VLMSs. Conclusions . The observed differences in the chemistry of the inner disk as a function of host properties are likely to be accounted for by differences in the disk temperatures, stellar radiation field, and the evolution of dust grains.